1. Field of the Invention
The present invention relates to a scanning electron microscope which scans a beam of electrons on a sample, and detects electrons emitted from the scanning spot. Particularly, the present invention relates to a scanning electron microscope which includes decelerating-electric-field forming means for reducing the energy reaching the sample, and which is suitable for detecting backscattered electrons or secondary electrons emitted from the scanning spot.
2. Description of Related Art
A scanning electron microscope forms an image of a sample in accordance with the following scheme. The scanning electron microscope accelerates electrons emitted from a source of electrons, and thereafter focuses electrons by use of an electrostatic lens or an electromagnetic lens, thus turning the focused electrons into a narrowly-focused beam of electrons (a primary beam of electrons). Hence, the scanning electron microscope scans the beam of electron on a sample, and thereby detects electrons emitted from the sample, hence displaying the strength of the signal thus detected On a displaying apparatus in synchronism with the scanning of the beam of electrons.
The electrons emitted from the sample by irradiating the beam of electrons on the sample have a wide energy distribution. Parts of the primary beam of electrons irradiated on the sample are elastically scattered by atoms positioned in the surface of the sample. These are termed as backscattered electrons (hereinafter shortened to “BSEs”, and also hereinafter referred to as “reflected electrons” in some cases). It is known that most of the BSEs has the same level of energy as the beam of electrons.
In addition, other parts of the beam of electrons interact with atoms positioned inside the sample. As a result, electrons positioned inside the sample receive kinetic energy, and thus are emitted from the sample. These are termed as secondary electrons (hereinafter shortened to “SEs”). It is known that most of the SEs has an energy of less or equal to 50 eV, or of approximately 10 eV on the average.
Out of the electrons emitted from the sample, the BSEs carry information on the composition of the sample. While, the scanning electron microscope is required to meet needs for doing such as analyzing the composition by selectively detecting the BSEs. Scanning electron microscopes which selectively detect BSEs or distinguish BSEs from SEs are described respectively in Japanese Patent Application Laid-open Publication No. 2004-221089 (hereinafter referred to as “Patent Document 1),” Japanese Paten Application Laid-open Publication No. 2002-324510 (hereinafter referred to as “Patent Document 2),” International Publication No. WO00/19482 (hereinafter referred to as “Patent Document 3),” Japanese Patent Application Laid-open Publication No. Hei 8-273569 (hereinafter referred to as “Patent Document 4)”.
Patent Document 1 describes a configuration of the scanning electron microscope which has a aperture arranged in a vicinity of the crossover of BSEs and has a detector arranged between the aperture and the source of electrons, as well as which thus detects a signal in the center of the BSE.
Patent Document 2 describes a configuration of the scanning electron microscope which has a detector arranged off the optical axis of the beam of electrons, and which thus applies a positive voltage of approximately 10 kV to a vicinity of the detector in order to guide secondary electrons.
This configuration guides the SEs to the detector due to the strong electric field. However, the trajectories of BSEs are not bent so much because of their high energy. As a result, the BSEs travel straight toward the source of electrons.
Patent Document 3 describes a configuration of the scanning electron microscope which has a secondary electron conversion electrode in the trajectories of BSEs, and which switches between the detection of SEs and the detection of BSEs by doing such as changing voltages applied to the secondary electron conversion electrode. Here, the trajectories of BSEs are changed by an electromagnetic objective lens.
Patent Document 4 describes a technology which causes the scanning electron microscope to separate the trajectories of SEs from the trajectories of the BSEs by use of the focusing effect of each corresponding lens, as well as which detects the BSEs by use of a BSE detector arranged inside the optical axis of the beam of electrons, and detects the SEs by use of a SE detector arranged outside the optical axis of the beam of electrons.
Japanese Patent Translation Publication No. Hei 9-507331 (hereinafter referred to as “Patent Document 5) describes a technology which decelerates the landing energy of a beam of electrons (or an energy which the beam of electrons has when reaching a sample) by applying a negative voltage to the sample. In addition, this patent document describes a technology to selectively detect BSEs by use of a detector which is arranged above the objective lens (closer to the source of electrons). Because a negative voltage is applied to the detector for detecting BSEs, SEs are repulsed back to the sample due to their lower energy than that of the BSEs. As a result, the BSEs are selectively detected.
Japanese Patent Application Laid-open Publication No. Hei 9-171791. (hereinafter referred to as “Patent Document 6) also describes a technology which decelerates the landing energy of a beam of electrons by applying a negative voltage to the sample (this technology is sometimes termed as a retarding technology). This technology makes it possible to decrease the energy of the beam of electrons which reaches the sample, with the energy of the beam of electrons kept at a high level while the beam of electrons passing the object lens. This enables the scanning electron microscope to have a higher resolution, and to cause less damage on the sample, at the same time.